The deployment and use of wireless communication systems is dramatically increasing, with associated pressures to increase system capacity, bandwidth, and quality. One way to increase system capacity is by employing a multiple access process, which allows a scare system resource to be shared between multiple users. For example, some communication systems operate according to a TDMA (time division multiple access) process. In a TDMA communication system, a predetermined time frame is sub-divided into slots, and each user is assigned a slot for receiving and transmitting data or voice signals. In this way, multiple users are able to share the same time frame.
In another example, some communication systems operate according to an SDMA (spatial division multiple access). In an SDMA system, a directable antenna array is configured to allow a particular frequency to be used by multiple users operating in the same general geographic area. In use, each mobile user operates a device that is assigned a unique word prior to initiating data or voice communication with a base station. Thereafter, from time to time, the unique word is transmitted from the remote user, which assists the base station in determining the spatial signature of the remote device. In turn, this allows the base station to configure its directable antenna to better differentiate communication signals originating from different mobile devices, even though the devices are communicating on the same frequency. In this way, an SDMA communication system allows multiple uses to share the same frequency.
Some communication systems may use more than one multiple access process to further increase system capacity. For example, the PHS (personal handyphone system) communication system, which is widely deployed in Japan, combines the benefits of both TDMA and SDMA. That is, PHS divides a time frame into slots, and then assigns unique words with respect to each slot. In this way, each time frame allows for multiple users in the slots, and each slot allows for multiple access by using the same frequency. In PHS, the base station is generally referred to as the cell station, while the remote mobile device is referred to as the personal station.
The PHS system is a recognized international standard promulgated by ARIB (Association of Radio Industries and Businesses). More particularly, document RCR-STD-28 details the requirements and options available in a PHS communication system. For example, PHS, as with other SDMA communication systems, may be implemented with a limited number of available unique words. Although the unique words may be selected for low cross correlation effects, because there are a limited number available, unique words are reused throughout the PHS communication system. However, to enable the directable antenna to operate properly, it is important that the unique words for personal stations be different, and more importantly, to be different enough to support signal differentiation by the cell station. Accordingly, when a personal station makes a request to access the PHS cell station, the PHS cell station should consider which unique words are in use in an area around the requesting personal station.
Currently, there are two known techniques for attempting to avoid assigning the same unique word to two personal stations operating in the same general area. However, neither has proved satisfactory. First, the cell station may monitor the communication traffic channel (“TCH”) being transmitted from neighboring cell stations to determine which unique words are currently in use in adjacent cells. However, monitoring the TCH channel requires signification system resources, and may degrade overall system capacity. Also, cells may not overlap sufficiently to allow a complete monitoring and accounting of all or nearly all of the unique words in use. Accordingly, monitoring TCH to assist in assigning unique words has not proved practical. Second, each cell station may attempt to randomize its assigning of unique words. However, even when each cell station randomly assigns words, it is possible to assign the same or similar unique words to personal stations operating in the same general geographic area. In this regard, two personal stations assigned such similar unique words will generate communication signals that are undesirably similar, and overall system capacity and quality will be reduced. Stated differently, it is desirable that the signals generated by such mobile stations have a low level of correlation to allow for better signal differentiation.
Therefore, there exists a need for a process and system for assigning unique words to achieve an improved system capacity, to lower interference, and to enable better signal differentiation.